1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and a nonvolatile memory system which are configured using electrically rewritable nonvolatile memory cells.
2. Description of the Related Art
An EEPROM (Electronically Erasable and Programmable Read Only Memory) is mounted in various pieces of equipment as an electrically rewritable nonvolatile semiconductor memory device. Recently, various kinds of multi-level memory systems in each of which one nonvolatile memory cell (hereinafter, may be referred to merely as a “memory cell”) memorizes multi-bit data are proposed in order to realize capacity enlargement (see, for example, a patent document 1). The multi-level memory system divides an extensive voltage region ranging from a low voltage region to a high voltage region into many voltage regions, and uses the divided voltage regions as threshold values of the memory cell. When the threshold value is set in a higher voltage region, writing needs to be carried out by a higher writing voltage.
When a solder or others is used in a packaging process of fixing the semiconductor memory device like this to a package, a stress by high temperature about 200° C. may be exerted on the semiconductor memory device. An erroneous reading fault that data written into the semiconductor memory device before the packaging process cannot correctly be read after the packaging process because of passing through the high temperature process may be caused. It is considered that the erroneous reading fault like this is caused because a part of charge upon being written into the semiconductor memory device leaks because of a stress by high temperature and a threshold value written into a cell is lowered compared with that just after the writing. For this reason, it is required to take measures considering that the threshold value of the written data is lowered in the packaging process. In addition, a similar erroneous reading fault tends to easily arise because of advance of micronization as well as the multi-level memory system, and the measures are required.